Pyrolitic graphite coated casting mold and method of making same



Nov. l5, 1966 w. H. scHwElKERT 3,284,862

PYROLYTIC GRAPHITE COATED CASTING MOLD AND METHOD OF MAKING SAME Filed May e, 1964 INVENTOR. W/W/P A( .QZ iff/KEW' irravf/ United States Patent O 3,284,862 PYROLITIC IGlRAPHITlE COATED CASTING MOLD AND METHOD OF MAKING SAME Wilbur H. Schweikert, Cincinnati, Ohio, assignor to General Electric Company, a corporation of New York Filed May 6, 1964, Ser. No. 365,255 7 Claims. (Cl. 22-192) This invention relates gene-rally to refractory casting molds and their production, and more particularly, to an improved casting mold useful in the p-recision casting of reactive and refractory metals and alloys.

Because of the chemical reactivity ibetween molten metals such as Ti, Zr and Hf found in Group IVb of the Periodic Table of Elements, the casting art has developed the use of isotropic graphite molds. The practice has become attractive for the relatively small amount of casting now ybeing conducted with reactive metals such as Ti as well as with refractory metals and their -alloys such as those based on molybdenum, columbium, chromium, tungsten, tantalurn and the like.

Such known graphite molds have been made from powdered or granular graphite and a carbonizable binder which together is forme-d into a green mold and then subsequently cured. Other molds are made by machin-ing shapes in graphite blocks or by compressing pattern shapes into gnaphite. Still others have placed isotropic graphite on the -face of green molds prior to firing. However, using known methods and conventional isotropic graphite to make molds, the cost is relatively `high and it is difticult if not impossible to produce the complex type of precision casting molds such as are commonly rnade of refractory mater-ials for the well known lost wax process. In addition, and very important, appreciable solution or carbon pick-up in the metal is experienced through the use of ordinary isotropic graphite.

Of the reactive and refractory type metal and metal alloys mentioned above, titanium and its alloys have been most widely -used and cast into graphite molds. Although a number of methods of making graphite molds have been reported, the Ibest method to date for production purposes has ybeen to machine ordinary isotropic graphite into a mold. However, these molds are very expensive to make and cannot be used for very complex or complicated shaped articles which woul-d be preferably made of titanium.

The potential use is very good for titanium as intricately shaped high quality articles of a variety of sizes. However, prior to the present invent-ion, attempts to precision cast titanium into complex molds have resulted in castings of questionable quality, principally because of reaction between molten titanium and the mold material, or extremely high costs when machined molds are used. Hence titanium castings have fgreatly lagged potential applications.

It is a principal object of the present invention to provide a casting mold Imade by conventional precision cast` ing mold manufacturing methods, t-he internal wall surfaces of which include a coating of pyrolytic graphite.

Another object is to provide a method for making such a pyrolytic graphite coated mold from an ordinary refractory precision casting mold.

These and other objects and advantages will be more readily understood from the following detailed description, examples and the drawing which is a diagrammatic, partially section View of apparatus used in the practice of the present invention.

Brieliy, the present invention, in one form, provides a casting mold, which is commonly a hollow precision shell mold of the refractory material type, having inner walls defining a hollow inte-rior into which metal is cast, the

ice

inner walls including a continuous coating of pyrolytic ygraphite adjacent the hollow interior. In its method form, the present invention provides, in a method of making such a casting mold, the step of depositing by pyrolysis a continuous coating of pyrolytic graphite on the inner walls to be contacted by the cast metal.

In recent years, a highly oriented crystalline anisotropic ,graphite material called pyrolytic graphite has become of interest. It has been widely studied and reported particularly in connection with space technology and materials because of its unusual high temperature strength and erosion resistance, and unusual thermal and electrical properties. Although pyrolytic graphite and the ordinary graphite used in the known molds mentioned above are -both a form of carbon and generally of about the same colo-r, the-re is a great ditference in this physical and mechanical characteristics, and their grain and crystal orientation. For example, the grain orient-ation ratio for ordinary commercial isotropic graphite may be as high as 3 to 1. However, the anisotropic pyrolytic graphite has an orientation ratio between -to-1 and 2000 or more to l. Thfus pyrolytic graphite has long stran-d-like crystals, which are generally free of cracks, oriente-d in a single plane. Its practical production is by the dissociation of a hydrocarbon vapor or gas at very low pressures-less than one atmophere-and at temperatures up to as high as about 4500 F. The rate of deposition depends on the type of hydrocarbon, the temperature and the amount of vacuum p-resent. The resultant deposition on a surface is in a ser-ies of laminations substantial-ly parallel to the surface on which it is formed.

As a result of its special orientation, heat travels h-undreds of times more easily along the layers or laminations than through them. Consequently in one direction, pyrolytic graphite can act as a thermal con-ductor and in another -direction `as a thermal insulator. Crystals of ordinary isotropic graphite are arranged at random such as by pressure forming or through a binder so that .thermal conductivity is a function -of thickness. Pyrolytic graphite is significantly stronger and more corrosion and erosion resistant than is ordinary graphite. These and other differences between pyrolytic `graphite and ordinary graphite have been widely described in the literature.

High quality precision casting .molds of refractory materials have long been produced by the well known lost wax or investment casting technique. It was therefore thought desirable by some to place iinely divi-ded .graphite powder into the green mold kface during practice of such casting methods with the idea that suilicient graphite would remain after mold tiring to provide a non-reactive surface for molten titanium ror other reactive metals subsequently to be cast into the mold. However, the most common and efiicient practice in these 'precision casting techniques is to re the green mold in a high temperature furnace in air, for example, as high as 1800 F., to be certain that al1 carbonaceous material from the pattern wax has been burned away. Under these conditions, the graphite in the surface of the green mold would b-e oxidized as well. Therefore, the present invention of providing a pyrolytic 'graphite coating on the inside of an already made precision casting mold produced by commercially available, widely used, well known techniques allows the casting of `reactive or refractory metals in a practical, e'lcient and economical manner. Furthermore, pyrolytic graphite has 'been found to .be less susceptible to attack Iby molten metal as compare-d with ordinary, isotropic graphite. In fact, through the use of known isotropic graphite molds, severe carbon contamination has been experienced in the cast-ing of titanium articles. Therefore, and very important to the article, it 'has been found that through the use of pyrolytic graphite,

B a relatively small amount of carb-on is picked up by the alloy cast into the mold.

It has been found that deposition of pyrolytic graphite on the internal surfaces of precision refractory molds such as made by the lost wax process can be accomplished readily with known equipment using the present invention, at pressures between 1 mrn. Hg and less than l latmosphere and at tem-peratures between l300-22 00 F.

depending on the dissociation temperature and deposition rate of the hydrocarbon and depending on the weakening point or utility strength range of the mold binder. One type of apparatus which is relatively simple and has been used successfully is shown diagrammatically in partial section in the drawing. A mold which is a conventional ceramic shell mold made around a normal lost wax investment, is placed on a graphite base plate 12. The cup 14 of the moldis sanded flat for good contact with the base plate. A putty-like material such as one made of a mixture of graphite and a resinous binder, is troweled around the outside of the cup at 16 as a seal to insure a gas tight fit so that gas introduced through inlet 18 does not leak out of the cup and ow up around the outside of the mold.

The mold is surrounded by a graphite susceptor 20 which makes an enclosure around the mold. S-usceptor 20 is inside an induction coil 22 and together they serve as a means to heat the mold and gas through radiation. A graphite cover 24 and outlet port 26 are placed over the mold. This entire arrangement is situated in a vacuum chamber, not shown, connected to a vacuum pumping system. Mold 10, which is designed to produce an article 28 shown in phantom, is made up of passages to open in the area of cup 14 and act as vents 30.

In practice, a hydrocarbon gas flows into the mold through inlet 18 at cup 14 and up through the mold sprue 32. The gas is free to flow from the sprue into the mold cavity and deposit pyrolytic graphite on all of the mold faces before passing out through vents 30. Unused gas then ows Iaround the outside of the mold and out through the top vent 26 into the vacuum chamber.

Example 1 Shell mold 10 of the drawing was made by the lost wax process from a commercially available ceramic shell mold material comprising silica powder, silica sand and a binder of sodium silicate. Such mold materials are commercially available under such trade names as Nalcast, Glascast, Mono-shell and the like. Frequently other binders such as the alginates or other silicates such as ethyl silicate are used. Other ceramic type shell molds such as based on alumina or zirconia are also available and are commonly used in the investment casting process.

In this specic example, the shell mold, made with the use of ordinary commercially available pattern wax, was de-waxed and cured at 1600-1800" F. for about 1 hour. Originally, the portions of the mold shown as vents 30 in the drawing were attached to cup 14. However, in order to provide a vent for hydrocarbon gas which was later to be pumped through :the mold during pyrolysis, that port1on of the mold was cut away from its attachment point. Then cup 14 was sealed with conventional mold dip coat. Thus vents 30 were provided in the mold as shown in the drawing.

The completed shell mold was placed in the vacuum furnace apparatus described in the specicaltion and a vacuum of about 0.4 mm. of Hg pressure was produced after the chamber was rst purged with argon. The mold was then heated to about l900 F. at which time acetylene gas was introduced through inlet 18 for passage through the mold and ou-t through vents 30 for exhausting through outlet 26 in the vacuum chamber. The acetylene flow rate was about cubic feet per hour at the 3 mm. Hg pressure. After about l hour at these conditions, a pyrolytic graphite coating of between 0.002-0004 was deposited umformly on the internal surface of the mold.

It is interesting to note the because of the porosity of this type of refractory mold, the dissociation gas, because of the lower vacuum outsidet he mold, rst seeped through the mold pores and permeated the mold material with a sooty type carbon. This fact was found by subsequent sectioning of the mold and indicated the preferred use of a porous mold rather than a non-porous one. This allows a small amount of gas to pass through the pores of the mold until each portion of the surface is sealed with soot after which a continuous coating of pyrolytic graphite is deposited. As some -of the pores are sealed, the gas will selectively seek other pores or passageways until the entire surface is completely lled. Thereafter, a uniform coating of pyrolytic graphite is produced in the stratied manner described before. Therefore, the use of at least a slightly porous mold is preferred and particularly advantageous in order to assure a more uniform coating.

The mold :of this example was used in the vacuum casting of a melt of commercially pure titanium metal. Examination showed no reaction between the mold and the article and virtually no carbon pickup.

Example 2 The same mold material and apparatus described in Example 1 above was used except that natural gas, principally methane, was introduced through inlet 18. Because of the higher dissociation temperature required for the pyrolysis of this type of gas, it was necessary to increase the temperature of the mold to about 2150 F. The gas was introduced at a rate of 12 cubic feet per hour under a pressure of about 3 mm. Hg. After about 1 hour at temperature, the mold was cooled and a pyrolytic graphite coating of about 0002-0004 was observed,

A variety of hydrocarbon gases or vapors can be employed in the pyrolytic deposition of anisotropic graphite, the limiting factors according to the present invention being the maximum temperature which the mold material, particularly the binder, can withstand and still be sufficiently strong to be usable for casting and the temperature at which the gas or vapor must be taken to dissociate for pyrolytic purposes. Acetylene gas as low as 1300 F. and methane as low as 1650 F. have been used successfully to deposit pyrolytic graphite. The deposition rate for each hydrocarbon varies with dissociation temperature and pressure. Pressures less than about 1 mm. Hg of gas have been found to be impractical for production because of the long deposition times.

Recently, doped pyrolytic graphite has been produced with small percentages of such elements as boron for increasing the strength. It is contemplated that these kinds of pyrolytic graphite coatings can be produced in the practice of the present invention.

Although the present invention has been described in connection with specific examples, it will be understoodby those skilled in the yart of metal casting and m-old preparation and production of pyrolytic graphite, the variations and modifications of which the present invention is capable.

What is claimed is:

1. An investment casting mold including:

porous inner walls defining a hollow interior into which v metal is cast;

the inner walls including a continuous coating of pyrolytic graphite adjacent the hollow interior.

2. A precision casting mold comprising:

a porous shell of a refractory material, the shell including inner wal-ls enclosing a hollow interior into which metal is cast; and

a continuous coating of pyrolytic graphite on the inner walls sepa-rating the inner walls from the hollow interior.

'3. A precision investment casting mold for the casting of titanium Iand its alloys comprising:

a, porous refractory shell of a materia1 which is reactive with molten titanium, the shell including inner S walls enclosing a hollow interior into which metal is cast; and

a continuous coating of pyrolytic graphite on the inner walls separating the inner walls from the hollow interior.

4. In a method of making a precision casting mold, comprising a shell including inner walls enclosing a hollow interior into which metal is cast, the steps of:

making an investment shell mold of a refractory material;

heating the mold ina vacuum at a temperature between 1300-2200 F. while at the same time passing through the interior of the mold at a pressure of 1-10 mm. Hg a hydrocarbon gas which will dissociate at that temperature and will deposit pyrolytic `graphite on the inner walls of the mold,

the refractory material being one which will not weaken at the heating temperature suicient to render the mold unusable for casting.

5. In a method of making a precision casting mold, comprising a shell including inner walls enclosing -a hollow interior into which metal is cast, the steps of:

making an investment shell mold of 'a refractory Inaterial reactive with molten titanium;

heating the mold in a Vacuum at a temperature between l900-2150 F. While at the stme time passing through the interior of the mold at a pressure of 1-10 mm. Hg acetylene .gas to deposit pyro-lytic graphite on the inner walls of the mold,

the refractory Imaterial being `one which will not weaken suiciently to render the mold unusable for casting after heating at 1900-2150 F.

6. In a method of preparing for casting a porous casting mold including inner walls defining a hollow interior into which metal in cast, the steps of z placing the porous mold ina vacuum furnace;

passing through the interior of the mold a hydrocarbon gas while at the same time heating the mold at a tempearture which will cause dissociation of the gas but will not weaken the mold as to make it unusable for casting;

the pressure of the gas within the mold being less than 1 atmosphere 'and being suiciently ygreater than the pressure outside the mold to cause passage of the gas through pores in the mold until such pores are sealed with products of the dissociated gas yand thereafter to deposit a continuous coating of pyroly-tic graphite on the inner walls.

7. In a method of preparing for casting a porous easting mold including inner walls dening a hollow interior into which metal in cast, the steps of:

placing the porous mold in a vacuum furnace;

passing through the interior ofthe mold a hydrocarbon gas While at the same time heating the mold 'at a temperature between 1300-2200" F. which will cause dissociation of the gas but will not weaken the mold as to make it unusable for castthe pressure of the gas within the mold being 1 1() mm.

Hg and being sui'liciently greater than the pressure outside the mold to cause passage of the gas through pores in the mold until such pores are sealed with :products of the dissociated gas and thereafter to deposit a continuous coating of pyrolytic graphite on the inner walls.

References Cited by the Examiner UNITED STATES PATENTS 1,492,694 5 1924 Meloche 249-111 2,245,651 6/ 1941 Craig et al. 22-192 2,681,485 6/1954 Smith. 2,862,826 12/1958 Hohn et al 10G-38.23 2,886,869 5/1959 Webb et al 22-216.5 3,138,435 6/1964 Diefendorf 23-209.1

FOREIGN PATENTS 1,3 18,796 1/1963 France.

References Cited by the Applicant UNITED STATES PATENTS 338,542 3/1886 McTighe.

OTHER REFERENCES Iron Age Magazine, Sept. 7, 1961, pp. 102-103, New Mold Methods Stir Interest in Reactive-Metal Casting.

J. SPENCER OVERHOLSER, Primary Examiner.

MARCUS U. LYONS, Examiner.

E. MAR, Assistant Examiner. 

1. AN INVESTMENT CASTING MOLD INCLUDING: POROUS INNER WALLS DEFINING A HOLLOW INTERIOR INTO WHICH METAL IS CAST; THE INNER WALLS INCLUDING A CONTINUOUS COATING OF PRYROLYTIC GRAPHITE ADJACENT THE HOLLOW INTERIOR.
 4. IN A METHOD FOR MAKING A PRECISION CASTING MOLD, COMPRISING A SHELL INCLUDING INNER WALLS ENCLOSING A HOLLOW INTERIOR INTO WHICH METAL IS CAST, THE STEPS OF: MAKING AN INVESTMENT SHELL MOLD OF A REFRACTORY MATERIAL; HEATING THE MOLD IN A VACUUM AT A TEMPERATURE BETWEEN 1300-2200*F. WHILE AT THE SAME TIME PASSING THROUGH THE INTERIOR OF THE MOLD AT A PRESSURE OF 1-10 MM. HG A HYDROCARBON GAS WHICH WILL DISSOCIATE AT THAT TEMPERATURE AND WILL DEPOSITE PYROLYTIC GRAPHITE ON THE INNER WALLS OF THE MOLD, THE REFRACTORY MATERIAL BEING ONE WHICH WILL NOT PYROLYTIC AT THE HEATING TEMPERATURE SUFFICIENT RENDER THE MOLD UNUSUABLE FOR CASTING. 